Typically, the 3rd Generation Partnership Project (3GPP), as one of the representative standardization organizations, has specified Evolved Packet System (EPS) with the introduction of Mobility Management Entity (MME) for managing mobility.
In such evolved mobile communication system, the NAS protocol used in the conventional 3G mobile communication system is employed as modified to meet the requirements for the next generation high speed packet communication service.
Meanwhile, IP multimedia Subsystem (IMS) is a framework for supporting Internet Protocol Multimedia service in which the fixed and mobile communication networks supporting the providing public communication services using different circuit or socket switches are integrated by means of the Session Initiation Protocol (SIP) as the protocol for use in IP technology and Internet telephony and has been standardized by 3GPP represented by W-CDMA and 3GPP2 represented by CDMA-2000. This technology is referred to as IMS in 3GPP as W-CDMA camp while referred to as Multimedia Domain (MMD) in 3GPP2 as CDMA2000 camp. IMS is advantageous in that the voice call service, as well as web browsing and e-mail services, can be controlled in the packet service domain with SIP as higher protocol. In spite of the mature standardization of IMS, the service launch is delayed because the bearer setup and call setup for IMS are not supported.
Since EPS is designed to support the packet switched (PS) services, in order to enable provisioning of the voice call service widely, it is necessary to fall back to a CS network according Circuit Switched Fall Back (CS fall back or CSFB) or, if the IMS service is supported, perform handover from 3GPP EPS to 3GPP2 CDMA200 1x or 3GPP UTRAN/GERAN In such a situation. Along with this situation, the discussion is under way about the emergency call service. However, the detailed procedure and properties for supporting the emergency call service, e.g. how to provide the unauthenticated terminal with the emergency call service and how to release the connection to the unauthenticated terminal after the end of the emergency call, is not defined yet. There is therefore a need of a method and system element for supporting emergency call service efficiently.